Europium(iii)/terbium(iii) mixed metal–organic frameworks and their application as ratiometric thermometers with tuneable sensitivity in organic dispersion

Abstract

The ability to engineer on a molecular level luminescent metal–organic frameworks (MOFs) enables the design of well-performing ratiometric, i.e., self-referencing, temperature sensors. Lanthanide-based MOFs stand out as luminescent temperature sensors due to their high luminescence intensity and the sharp emission lines of the lanthanides. The use of two different lanthanide cations, incorporated into the same MOF structure, is supposed to enable ratiometric temperature sensing. Herein, we present a series of mixed-metal Eu_xTb_(1−x)BTC, in which the metal ions are homogeneously dispersed, as demonstrated by ¹H solid state NMR spectroscopy. The Eu_xTb_(1−x)BTC series shows controllable luminescent properties, which depend on the solvation of the lanthanide. The two MOFs in the series with the lowest Eu contents, namely Eu_0.05Tb_0.95BTC and Eu_0.02Tb_0.98BTC, are suitable candidates for ratiometric temperature sensing, achieving sensitivities of up to 2.0% K⁻¹. As the fluorescence is affected by the presence of solvents, simultaneous ratiometric temperature and solvent sensing is possible with remarkable high thermal sensitivities of ca. 0.1% K⁻¹ and ca. 0.2% K⁻¹ for dispersions of Eu_0.02Tb_0.98BTC in acetonitrile and ethanol, respectively.